1. Field of the Invention
The present invention generally relates to a flash memory, and more particularly to uniform coding for a multi-level cell (MLC) flash memory.
2. Description of the Prior Art
Flash memory is a non-volatile solid state memory device that can be electrically erased and reprogrammed, and is a specific type of electrically erasable programmable read-only memory (EEPROM) device. Conventional flash memory stores a single bit of information in each memory cell such that each memory cell can be programmed to assume two possible states. The conventional flash memory is thus commonly referred to as single-level cell (SLC) flash memory or single-bit cell (SBC) flash memory. Modern flash memory is capable of storing two or more bits of information in each memory cell such that each memory cell can be programmed to assume more than two possible states. The modern flash memory is thus commonly referred to as multi-level cell (MLC) flash memory or multi-bit cell (MBC) flash memory.
In the flash memory, data of different state are written to the flash memory (which is commonly referred as programming the flash memory) by storing different amount of charge in the floating gate of the flash memory. As the charge in the floating gate specifically determines the corresponding threshold voltage, the data can then be read from the flash memory according to their different threshold voltage. Due to variations among the memory cells during the manufacture, operation or according to other factors, the threshold voltage of each state is not a constant value but a range. FIG. 1A shows a common distribution of the threshold voltage for a typical MLC flash memory (a two-bit cell flash memory is exemplified here). The entire voltage range is divided into a number of regions (e.g., four regions in the example), each region corresponding to one state. The number of cells of each threshold voltage is collected as illustrated. When the flash memory is being read, the threshold voltage of a cell is compared to reference voltages or read thresholds (e.g., V1, V2 and V3 in the figure) to determine its state. Specifically, the threshold voltage is firstly compared with a low-byte read threshold (e.g., V2 in the exemplary figure), followed by subsequently comparing with high-byte read thresholds (e.g., V1 and V3 in the exemplary figure).
However, partial distribution of threshold voltages may become widened, such as the state (0,0) shown in FIG. 1B, when the low-byte data and the high-byte data in the same word line have substantially the same value. As a result, the read margin decreases and error rate increases.
For the reason that conventional flash memory, particularly the MLC flash memory, could probably result in read errors due to widened distribution of threshold voltages of one or more states, a need has arisen to propose some novel schemes to cause the threshold voltages more uniformly distributed.